Thermochromic platinum complexes

ABSTRACT

Thermochromic compounds containing the [Pt(dipic)Cl] -  anion. These compounds are yellow and monomeric at high temperatures or in low concentrations and abruptly change to red and polymeric at low temperatures or higher solution concentrations. This unusual property allows them to be used as temperature sensors.

GRANT REFERENCE

This work was financed in part by the U.S. Department of Energy, Officeof Basic Energy Sciences, Chemical Sciences Division, under ContractW-7405-ENG-82. The government may have certain rights in the invention.

p This is a division of copending application Ser. No. 202,456, filed onJune 6, 1988, now U.S. Pat No. 4,857,231.

BACKGROUND OF THE INVENTION

In industry there are many instances where "temperature alert" signalsare needed. In many cases, when a temperature reaches above or below acertain value or a set range, this change must be observed and aresponse made. This often occurs in operation of chemical processes, instorage of packaged temperature-sensitive products, and the like.Moreover, there are many instances where it is simply impractical tohave sophisticated electrical sensors, either because such devices aretoo expensive or because of the skill needed for their operation andmaintenance.

There is therefore a real and continuing need for the development ofrhermochromic materials, which can change color as the temperaturechanges. They are similar to the litmus paper, used for testing theacidity or basicity of solutions. Moreover, there is also a continuingneed for the development of thermochromic materials whose color isresponsive over a wide range of pH, regardless of the other ions presentin the medium.

An ideal thermochromic material is one whose color changes abruptly,rather than gradually, so that a sudden change is immediately apparentto observers who are monitoring temperature. An ideal thermochromicmaterial is also one whose concentration can be varied in order toadjust the threshold temperature, at which the color changes. In theideal case, the variables affecting the temperature threshold can becontrolled. Here, this "alert temperature" can be varied by simplyvarying the concentration.

A yet further characteristic of an ideal thermochromatic material isthat it undergoes not only a color change, but also a phase change.Thus, observers who are monitoring the temperature to determine changecan immediately see not only color, but also phase change. Finally, thechanges in color and phase should occur suddenly and dramatically foreasy notice.

The primary objective of the present invention is to provide athermochromic material that is closer to the ideal material than thoseused before, in the sense that the "alert temperature" can be varied bysimply manipulating the concentration of the thermochromatic material.

A yet further objective of the present invention is to provide athermochromic material which not only has color change based uponsensing of temperature, but which also has phase change.

A still further objective of the present invention is to provide athermochromic material which is easy to prepare, and which providesdramatic, sudden, and reversible color and phase changes.

The method and manner of accomplishing each of these objectives as wellas others will become apparent from the detailed description of theinvention which follows hereinafter.

SUMMARY OF THE INVENTION

This invention concerns thermochromic complexes of platinum(II), whichcontain the anion [Pt(dipic)Cl]⁻. These complexes in aqueous solutionare thermochromic over a wide pH range and regardless of the cationspresent. The variation in temperature or in concentration causes twochanges, a change in color and a change in phase, which are sudden,concurrent, and reversible.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a graph showing the characteristic thermochromism and phasechange for the compounds of the present invention, and how those can bemanipulated to provide an alert for different threshold temperatures.

DETAILED DESCRIPTION OF THE INVENTION

The complexes of this invention are new, never having been prepared orused before. They are thermochromic platinum(II) complexes of theformula: ##STR1## In the formula, M is a metal cation, X is a halideanion, and Z is a whole number equal to the cationic charge of M.Aqueous solutions of these complexes have been found to have the highlyunusual property of being thermochromic over a wide pH range, regardlessof the cation present. Moreover, the variation in temperature or inconcentration causes two sudden and abrupt changes, one in color and onein phase, both of which are reversible. At lower concentrations andhigher temperatures, the aqueous solution of the thermochromaticplatinum(II) complexes is yellow and monomeric. However, at lowertemperatures or higher concentrations there is an abrupt and sudden andreversible color change to red and phase change to a gel-like state. Inparticular, as the color changes to red and the phase changes to agel-like state, the material abruptly polymerizes. Thus, at highertemperatures or lower concentrations the complex is soluble, yellow, andmonomeric, whereas at lower temperatures or higher concentrations it isinsoluble, red, and polymeric. As later explained, the thermochromicmaterial is formed by a reaction involving 2,6-pyridinedicarboxylicacid, often referred to here as "dipic" for brevity. The polymeric formis gel-like, and is red in color, and is microcrystalline, and iscomposed of stacked [Pt(dipic)Cl]⁻ units. Unlike other square-planarcomplexes that form stacks, the soluble form of [Pt(dipic)Cl]⁻ remainsmonomeric over a wide range of concentration and temperature until thesudden onset of polymerization. The nucleation and growth of the polymerare easily monitored owing to the concomitant color change. The squareplanar platinum(II) complex undergoes sudden polymerization by stackingand the resulting gel-like polycrystalline material is red, unlike theyellow solution.

As earlier stated, the complex undergoes the changes here describedregardless of the cation present; and the cation, M, may be monovalent,divalent, or trivalent. Suitable monovalent cations include lithium,sodium, potassium, cesium, and ammonium. Suitable divalent cationsinclude magnesium, calcium, and barium. Suitable trivalent cationsinclude aluminum and polyatomic cations of Group-V elements (nitrogen,phosphorus, arsenic) that form water-soluble salts. While any cationsare satisfactory it has been found that potassium seemed to work thebest and is therefore preferred. The sodium and the calcium salts didbehave somewhat what atypically in yielding precipitates, rather thangel-like materials, at low temperatures.

The symbol X in the earlier presented formula represents any suitableanion, preferably a halide, but it is believed that both cyanide andthiocyanat may also work. It is preferred, however, that X be a halideand most preferred that X is chloride. Chloride has been found mostsuitable for the reaction in preparing the thermochromic platinum(II)complexes from the anion 2,6-pyridinedicarboxylate, i.e.,"dipicolinate."

The thermochromic compounds of this invention are found to exhibit thecharacteristic thermochromism and phase change best in aqueoussolutions; these changes also occur in solutions that are predominantlyaqueous compositions. The compounds are soluble in water and theconcentration in the aqueous solution can range from about 5 mM to about200 mM, preferably from about 10 mM to about 60 mM.

FIG. 1, a phase diagram for the thermochromic compounds of the presentinvention, shows how both the color and the phase depend onconcentration and temperature of the solution. At conditions above theline, A, the compound is soluble, yellow, and monomeric. At conditionsof temperature and concentration below line B, the compound isinsoluble, red, and polymeric, consisting of the stacked [Pt(dipic)Cl]⁻anions and the corresponding M cations. At conditions between A and Bthe red and yellow phases coexist. The line A marks the temperature atwhich the first red spots appear on cooling, whereas the line B marksthe temperatures at which the last red spots disappear on heating at thesolution. Since the area between the lines A and B is narrow, thechanges are sudden and rapid. It can be seen from FIG. 1 thatconcentration of the solution can be varied to determine the thresholdtemperature at which the color change and phase change will occur.

The thermochromic platinum(II) complexes of the present invention may beprepared by a simple and direct reaction. In simplest description, thereaction is effected by mixing ammonium dipicolinate and potassiumtetrachloroplatinate, in equimolar quantities, in an aqueous solutionunder mild conditions. The complex is then purified by conventionalion-exchange chromatography. The reaction which forms the thermochromicplatinum(II) complex can be represented by equation (1). Various[Pt(dipic)Cl]⁻ salts with different cations can be prepared by cationexchange or, in the case of the bulky cations, by precipitation.##STR2## Since the acid dipicH₂ is sparingly soluble in water, thesubstitution reactions were carried out with its salts (NH₄)₂ dipic andK₂ dipic. The reaction between the anions PtCl₄ ²⁻ and dipic is slow atroom temperature. On heating, however, the reaction shown in eq 1 occursreadily.

Dipicolinic acid (2,6-pyridinedicarboxylic) acid is known and easilyavailable, for example, from Sigma Chemical Company. However, as earlierexplained, since it is sparingly soluble, it was converted to (NH₄)₂dipic and K₂ dipic.

The following examples are offered to further illustrate but not limitthe processes, compositions and uses of the present invention.

EXAMPLE 1

In the reactions shown below dipic acid was obtained from Sigma Chemicalas earlier explained. It was converted to (NH₄)₂ dipic by the followingprocedure. To a suspension of 1.00 g (6.0 mmol) of dipicolinic acid in 1mL of water were added to 20 mL (a large excess) of concentrated aqueousammonia. The mixture was heated at 60-70° C. and dried without charging.The white residue was dissolved in 5 mL of water and dried again at60-70° C. This procedure was repeated six times in order to remove NH₃completely. Yield, 1.10 g or 91%. Proton NMR spectrum (δ values) in D₂O: 8.62, t; 8.41, d; ³ J(H--H)=7.8 Hz.

Where K₂ dipic was used it was prepared in the following procedure. Asolution containing 0.109 g (0.65 mmol) of dipicolinic acid and 0.071 g(1.27 mmol) of KOH in 3 mL of water was dried at 60-70° C. to yield awhite residue. An excess of base was avoided lest it might causeunwanted side reactions with Pt complexes in subsequent experiments.Proton NMR spectrum (δ values) in D₂ O: 8.18, t; 8.10, d; ³ J(H--H)=7.6Hz.

The thermochromic complex was prepared in the following manner. Asolution containing 0.402 g (2.0 mmol) of (NH₄)₂ dipic in 40 mL of waterwas added dropwise to a stirred solution containing 0.830 g (2.0 mmol)of K₂ [PtCl₄ ]in 10 mL of water. The reaction is best carried out in thedark, or in dilute (ca. 0.25 M) HCl instead of water, in order tominimize formation of platinum metal and other side reactions. Themixture was heated at 80° C. for 4 h and its color changed from red toorange. The UV band at 272 nm, due to the free dipicolinate anion,decreased while a broad one at ca. 330 nm, due to the chelate complex,increased. The orange reaction mixture was cooled to room temperatureand the NH₄ ⁺ ions replaced with K⁺ ions on a column of CM 52 cationexchanger, sized 2.5×14 cm, that had been equilibrated with 0.10 M KCl.The same KCl solution was used as an eluent. Two bands formed. The firstone was red and contained the unspent K₂ [PtCl₄ ]; the second one wasyellow and contained K[Pt(dipic)Cl]. The yellow solution wasconcentrated in a rotary evaporator and the red or orange solid thatformed was filtered off, washed with ethanol and acetone, and dried in adesiccator. The average weight of the first crop from several syntheseswas 0.30 g; the yield (36%) was sacrificed for the sake of purity. Anal.Found (calcd. for C₇ H₃ NO₄ ClKPt): C, 19.31 (19.34); H, 0.99 (0.70); N,3.12 (3.22). (All elemental analyses were done by GalbraithLaboratories, Inc.) Proton NMR spectrum (δ values) in D₂ O: 8.45, t;7.92, d; ₃ J(H--H)=8.0 Hz. Infrared bands (cm⁻¹) in Nujol mull were asfollows. For the red salts: 3539 w, 3053 w, 2912 s, 1680 s mult, 1317 smult, 1151 m, 1103 m, 914 w, 831 w, 770 m, 752 m, 675 m, 596 w, 462 m,368 vw, 331 m, 219 m, and 300 m; for the orange salt: 3086 w, 3074 w,2926 s, 1668 s mult, 1323 s, 1150 m, 1136 m, 1111 m, 1106 m, 916 w, 831w, 768 m, 752 m, 675 m, 599 w, 463 m, 457 m, 370 vw, 332 m, 332 m, 301m, 263 vw, 247 vw, and 223 vw.

EXAMPLES 2-8

Salts: M [Pt(dipcic)Cl]_(z) : The cations K⁺ and NH₄ ⁺, which accompany[Pt(dipic)Cl]⁻ in the orange reaction mixture, were replaced with Li⁺,Na⁺, Cs⁺, Mg²⁺, Ca²⁺, Ba²⁺, and Al³⁺ on columns of Amberlite CG-52cation exchanger resin that were previously equilibrated with 0.10 Msolutions of the corresponding Cl⁻ or NO₃ ⁻ salts and washed with water.The amount of the resin used was greater than that required by the resincapacity and the quantities of the cations in solution. Elution with theequilibrating solution in each case produced the red band of the unspentK₂ [PtCl₄ ], which moved first, and the yellow one of the [Pt(dipic)Cl]⁻salt, which moved second. Only the yellow band was collected.

All the salts above, with the exception of the sodium and calcium saltsthat precipitated rather than formed a polymeric gel, yield pale yellowsolutions in water. When the yellow solution was cooled, but well abovethe freezing point, it turned to the gel-like substance. This substanceswas examined and found to actually be polymeric and microcrystalline instructure. At the beginning of the thermochromic transition, red spotsappear in the yellow solution; on further cooling at moderate rates,approximately 10 to 20 degrees Celsius per minute, the red spots growand merge rapidly until the former solution becomes an immobile,although not frozen, mass. All of the transformations were observed tobe reversible on heating. Although, as illustrated in FIG. 1, the colorchange occurs over a temperature range with some hysteresis, thethermochromic transition can be classified as sudden.

Variation of the solution concentration, again as illustrated in FIG. 1,at constant temperature causes the same color change as variation of itstemperature at constant concentration. When the yellow aqueous solutionis concentrated by evaporation, it suddenly turns into the same redgel-like substance as when it is cooled. Correspondingly when the redsubstance is added to water it gives a yellow solution, even at the sametemperature. Evidently the color change can be effected by eithervertical or horizontal transitions in the phase diagram shown in FIG. 1.Some studies with different buffers spanning the pH range of from 2 to9.6 were conducted. These qualitative experiments permitted the generalconclusion that the thermochromic transition and the phase change occurin the weakly acid and neutral solution alike. Moreover, it was observedthat the platinum(II) complex is unaffected by the presence of otherions. In particular, the solution of the pure complex and the reactionmixture (which also contained ammonium, chloride and tetrachloroplatnateions) behaved identically upon heating and cooling and gave similardiagrams of the type shown in FIG. 1. It is also clear from experimentsthat the thermochromism occurs only in solvents that are mostly aqueousin composition.

Finally, as illustrated in FIG. 1, since the change between the red andyellow forms occurs over a relatively narrow temperature range, andsince this range can be set simply by adjusting the concentration of thesolution, the platinum(II) complex salts of this invention promise to beexcellent temperature indicators. They may be especially useful inapplications when deviations from a certain "alert temperature" must bedetected quickly. Such examples may include packaging objects of art,where the packaged products are highly temperature-sensitive; storage ofperishable foods; control of workplaces and of other enclosed areas;etc. It therefore can be seen that the invention accomplishes at leastall of its stated objectives.

What is claimed is:
 1. A method of sensing temperature change,comprising:(a) preparing an aqueous solution of a thermochromicplatinum(II) complex of the formula: ##STR3## wherein M is a cation, Xis a halide, and Z is a whole number equal to the positive charge of M;(b) placing said aqueous solution in an environment in which the complexis monomeric and yellow in solution at a given temperature, but is redand a polymeric at a different temperature; (c) observing the color ofsaid thermochromic complex to determine whether a particular "alerttemperature" or a particular temperature range has been reached orpassed.